An article published in Newsweek magazine on Monday claims that the Central Intelligence Agency knew that two men suspected of links to al-Qaeda were in the United States months before they took part in the suicide attacks of 11 September.

Some US counterterrorism officials say [this] may have been the most puzzling, an devastating, intelligence failure

Newsweek

Under the headline, "The terrorists the CIA should have caught", the report argues that the CIA tracked one of the men, Nawaf al-Hazmi, shortly after he attended an al-Qaeda meeting in Malaysia in January 2000, but failed to alert other US law enforcement agencies.

CIA agents also discovered that another man, Khalid al-Mihdhar, had already obtained a multiple-entry visa that allowed him to enter and leave the US freely, Newsweek reports.

On the 11 September, the two men boarded one of the four hijacked airliners - American Airlines Flight 77 - and crashed it into the Pentagon.

Part of this goes right to the heart of communication between the various intelligence agencies. It has not been a flow of information when people needed it

Richard Shelby, Republican Senator

The Newsweek report alleges that had the FBI had the information, its agents could have uncovered the terrorist plot.

It comes only days after the White House had admitted that President Bush was told a month before 11 September of a plot to hijack American planes by Osama Bin Laden.

'Kept in the dark'

Newsweek also says that the CIA did not pass on the intelligence to the Immigration and Naturalisation Service, which could have stopped them entering the US.

CIA chief George Tenet has already faced severe criticism over 11 September

Instead, the two men lived openly in the US and used their real names to open bank accounts and take flying lessons.

The report says that the State Department even renewed Mihdhar's visa in July 2001 despite the CIA's having linked him to one of the men believed to be behind the attack on the USS Cole in Yemen in 2000.

The magazine says the CIA had files on the two men at its Counterterrorism Centre for a year and nine months.

It was only three weeks before the 11 September attacks, when CIA Director George Tenet ordered an urgent review of the files after receiving repeated signals of an imminent terrorist attack.

'Detection unlikely'

FBI Director Robert Mueller - whose agency is facing criticism over its handling of intelligence reports prior to last year's attacks - expressed doubts that the attacks could have been detected.

Mueller: acknowledged the FBI's analytical capability was "not where it should be"

"I don't think it is at all likely," Mr Mueller told NBC's Meet the Press on Sunday.

Last week though, Mr Mueller appeared to acknowledged that investigators might have been able to uncover part of the plot that led to the 11 September attacks.

Mr Mueller said if all the clues had been put together, "who is to say" what could have been discovered.

This followed a warning from one of its field agent in Minneapolis who complained that FBI headquarters in Washington ignored information about Zacarias Moussaoui, who is alleged to have been involved in planning the 11 September attacks.

In May, the White House had also admitted that President Bush was told a month before 11 September of a plot to hijack American planes by Osama Bin Laden.

"Part of this goes right to the heart of communication between the various intelligence agencies. It has not been a flow of information when people needed it," Richard Shelby, a Republican Senator, told NBC's Meet the Press.